Because the conventional electrochemical device comprising liquid electrolyte has stability problems such as electrolyte leakage and explosion, an electrochemical device using polymer electrolyte has been developed. For example, lithium-polymer battery comprising polymer electrolyte has many advantages over to the conventional ones in that it is superior in stability, more economical due to its improved charge-discharge efficiency, can be designed into various shapes and be manufactured in the form of a thin-film, thus enabling to reduce the battery size.
In particular, polyalkylene oxide (PAO)-based solid polymer, which has been widely used as polymer electrolytes, and gel-type polymer electrolytes, which comprise organic liquid electrolytes in the polymer, have been of great concern as polymer electrolytes in the field of lithium secondary battery. Polyalkylene oxide of low molecular weight or an organic solvent has been added as a plasticizer to increase the conductivity of polymer electrolyte. However, it has drawbacks that the physical properties of polymer electrolytes may be deteriorated or the stable gel electrolyte cannot be prepared when its plasticizer content is increased.
U.S. Pat. No. 4,830,939 and J. Electrochemm. Soc., 145, 1521 (1998) disclose a process of preparing a crosslinked polymer electrolyte by means of UV or electronic beam irradiation by using a mixture of an ion conductive liquid comprising a polyalkylene glycol compound having a chemically crosslinkable group and an electrolyte salt.
U.S. Pat. Nos. 5,830,600, 6,511,772 and 6,746,794 disclose the use of a fire-retardant additive for a non-aqueous electrolyte solvent to improve the thermal stability of a lithium secondary battery. A phosphate-based compound was used as the fire-retardant additive.